Diffusion pumps



March 21, 1961 I R. GELLER ETAL DIFFUSION PUMPS 2 Sheets-Sheet 1 Filed 'Aug. 2, 1956 March 21, 1961 R. GELLER ETAL DIFFUSION PUMPS 2 Sheets-Sheet 2 Filed Aug. 2, 1956 DTFFUSIGN PUMPS Richard Geller, Gif-sur-Yvette, and Guy Mongodin, Clamart, France, assignors to Commissariat a lEnergie Atomique, Paris France, a French society The present invention relates to difiusion pumps.

Dilfusion pumps are commonly used at the present time for obtaining a high vacuum. Heretofore the operation of these pumps was based upon the use of a hot vapor (oil, mercury, etc).

Briefly stated, the operation of these pumps is as follows. The fluid used in the pump is heated and injected in the form of a hot vapor blast into a space at ordinary temperature where it condenses, producing the desired reduction of pressure.

The use of such an apparatus involves some drawbacks inherent in the principle thereof.

'In particular, a source of calories is necessary to heat the fluid and this heating involves a substantial inertia when the pump commences its operation.

Furthermore, the rate of pumping, which is fixed, the corrosion of the pumping fluid (oil, mercury) by the substances that are pumped and its oxidation (which occurs readily in the hot state When air penetrates into the apparatus) constitute very serious drawbacks.

The difiusion pump according to our invention is characterized by the injection of pumping fluid in the form of a blast of cold vapor, that is to say a vapor at ordinary temperature (approximating 15 C.) and eflecting its condensation by employing a cooling liquid the boiling temperature of which under atmospheric pressure is sufliciently low to achieve a saturated vapor pressure of the pumping fluid at the said temperature which is lower than the pressure to be obtained, the saturated vapor pressure of said pumping fluid at ordinary temperature being higher than 1 mm. of mercury. The pumping fluid to be fed to the pump is kept at an ordinary temperature in a closed vessel so that its saturated vapor pressure at said ordinary temperature serves to project the vapors of this fluid into the ejector of the pump.

The operation of the pump according to our invention is as follows:

A primary vacuum is produced in the pump by means of the conventional primary pump which constitutes one of the elements of a difiusion pump. Communication between the closed vessel containing the pump fluid and the pump ejector is then efiected. The pump commences immediate operation, producing a molecular vacuum in the container to be exhausted.

The principal advantages of the pump according to our invention are:

The pump commences immediate operation.

A high rate of pumping is possible because a high flow rate of cold pumping vapor through the ejector may be obtained as a result of the pressure existing in the closed vessel containing the pumping fluid;

The possibility of adjusting this rate of pumping to any value within a given range;

Absence of corrosion of the pumping fluid employed; and

The fact that the pump is but little affected by an inflow of air thereinto.

i; Patent According to preferred embodiments of our invention, the pumping fluid consists of water vapor, carbonic acid gas or Freon (dichlorodifluoromethane). Water vapor need not be recovered. Carbonic acid gas is cheap and is easily obtainable. Freon is very inert from a chemical point of view.

In a general way, we employ as pumping fluid any fluid which has a saturated vapor pressure higher than 1 mm. of mercury at ordinary temperature and the saturated vapor pressure of which at the boiling temperature of the cooling liquid under atmospheric pressure is lower than the final pressure to be obtained, provided that this fluid is not incompatible from a chemical or other point of view.

Preferred embodiments of our invention will be hereinafter described with reference to the accompanying drawings, given merely by way of example and in which:

Fig. 1 is a diagrammatical view of a diflusion pump made according to our invention.

Fig. 2 shows a modification of such a pump.

In both of these embodiments of our invention, the pumping fluid consists of Freon 12.

Freon 12, or dichlorodifluoromethane, is a compound which is practically inert at temperatures lower than 200 C. Its boiling point at atmospheric pressure is 29 C. and it solidifies at the same pressure at -l58 C. The saturated vapor pressure of Freon at ordinary temperature (15 C.) is 4 atmospheres. Its saturated vapor pressure at -l C., i.e. the boiling temperature of nitrogen (which constitutes the cooling liquid) is very low (below 10- mm. of mercury), so that this compound is very suitable as pumping fluid in a diffusion pump according to our invention.

In an embodiment of our invention illustrated by Fig. 1, the primary pump (which may be of the type used in conventional diffusion pumps) is connected through a pipe 7 with the central duct 1. The body 2 of the diffusion pump, which communicates with the container 18 to be exhausted, is surrounded by a Dewar flask 3 containing liquid nitrogen. The pumping fluid (Freon) is contained in a closed vessel 14 which is connected through a pipe 5 with an annular ejector 8 surrounding the central duct 1. The flow of Freon from vessel 14 and the annular ejector 3 can be controlled by means of a valve 6, for instance a needle-valve. Advantageously, a pressure relief device (not shown) is interposed between vessel 14 and valve 6.

A primary vacuum is first obtained through pipe '7 by the primary pump, and the diffusion pump starts operating as soon as Freon is allowed by valve 6 to flow to ejector 8. The annular shape of this ejector achieves a good distribution of the vapor blast. The molecules of =Freon are cooled and condensed by the cold Wall of the pump body '2. in contact with liquid nitrogen. They do not immediately take the temperature of this wall so that the Freon drips in liquid form into the bottom of the pump body 9 where it partly solidifies.

The inertia of the pump is practically negligible and since it instantaneously starts operating, in a very short time a vacuum of 10- mm. of mercury is obtained in container 18.

Air may leak in occasionally into the pump body 2 Without producing any corrosion of the pumping fluid, when corrosive products are to be pumped off from container 18.

It was found that the pumping rate may be as high as 35 liters per second under pressures ranging from 10" to 10- mm. of mercury. This result is about twice that which may be expected from a single stage hot vapor diifusion pump of the same size.

Furthermore, owing to the provision of valve 6 it is possible to adjust the pumping rate at any value from to the maximum pumping rate.

Owing to these properties of our pump, it is possible to use it for pumping off corrosive fluids such as uranium fluoride.

The fact that the pump starts working instantaneously and the absence of any heating element are particularly advantageous in an apparatus intended to work intermittently.

Owing to the possibility of adjustment by means of valve 6 it is not necessary, as in the case of hot vapor diffusion pumps, to provide a special pump for every de sired value of the pumping rate.

The pumps according to our invention have the advantage that they can be opened as soon as Working under vacuum is finished.

Such pumps are very advantageously used as booster pumps.

In the construction of Fig. l, the consumption of fuel is relatively high because the Freon that has been used is not recuperated, and the consumption is about 1 liter of liquid Freon for 9 hours of operation.

It would appear that circulation of Freon in a closed circuit as in the conventional diflusion pumps would be difficult to obtain due to the high value (4 atmospheres) of the saturated vapor pressure of Freon at ordinary temperature. It might be endeavoured to keep the vessel 14 which contains liquid Freon not at ordinary temperature but at a relatively low temperature such as 50 C., where the saturated vapor pressure of Freon is only some millimeters of mercury. However, such an arrangement would complicate the construction of the pump and would reduce the efficiency of the high pressure blast from the ejector.

In order to obviate these drawbacks, we have provided an arrangement as illustrated by Fig. 2. The pump proper is similar to that of Fig. l, but liquid Freon instead of accumulating at the bottom of the pump body itself flows into a magazine vessel 10, also immersed in a Dewar flask 11. Between the pump body 2 and vessel 10 there is provided a fluidtight removable connection 12 and a valve 13. After the pump has been working for some time, vessel 10 is filled with Freon and valve 13 is closed and connection 12 removed. We then substitute for this vessel it] an identical empty vessel 10 which is similarly connected with the pump body 2. The vessel 10 full of Freon can then be substituted for the initial vessel 14 connected through 16 and valve with pipe 5, since during the above mentioned period this vessel 14 has delivered all the Freon it contained. With such an apparatus the consumption of Freon is reduced. Of course, in this case the Dewar flask 3 of Fig. 1 is to be replaced by a special Dewar flask 17 provided at its bottom with a passage for connection with part 12 of vessel 10.

In both of the above embodiments, the pump body is made of glass. Vessels 1t) and 14 must be made of a suitable metal to be able to withstand the 4.atmosphere pressure therein. Valves 13 and 15 are also metallic. Fluidtight connections 12 and 16 consist of two flat flanges, screwed together, and one of which is provided with a groove in which is housed apacking joint. This joint is advantageously made of a plastic material in order to resist the action of cold. 7 x

In a general manner, while we have, in the above description, disclosed what we deem to. bepractical and eflicient embodiments of our invention, it should be well understood that we do not wish to be limited thereto as there might be changes made in the arrangement, disposition and form of the. parts without departing from the principle of the present invention as comprehended within the scope of the accompanying claims 1. A high vacuum diflusion pump apparatus, including a primary pump, said diffusion pump. comprising a pump body having one end connected with a container to be exhausted, cooling means enclosing said pump body, a primary vacuum duct extending longitudinally into the central part of said pump body, first pipe means connecting said duct with the primary vacuum pump, an annular ejector mounted around said duct, 2. primary vessel for holding pumping fluid under pressure, second pipe means connecting said vessel with said ejector, said ejector directing the stream of said pumping fluid in a direction away from the container to be exhausted, means for detachably securing said vessel to said second pipe means, and valve means in said second pipe means for controlling the rate of flow of fluid to said ejector.

2. A high vacuum diflusion pump apparatus including a primary pump, said diffusion pump comprising a pump body having one end connected with a container to be exhausted, cooling means enclosing said pump body to form a cooling jacket thereabout, said cooling means comprising a Dewar flask and liquid nitrogen therein, a primary vacuum duct extending longitudinally into the central part of said pump body, first pipe means connecting said duct with the primary vacuum pump, an annular ejector mounted around said duct, a primary vessel for holding pumping fluid under pressure, second pipe means connecting said vessel with said ejector, said ejector directing the stream of said pumping fluid in a direction away from the container to be exhausted, means for detachably securing said vessel to said second pipe means, pumping fluid in said vessel, said pumping fluid being in the form of a vapor at ambient temperature and under atmospheric pressure and having a saturated vapor pressure higher than 1 mm. of mercury, which pressure can be reduced by cooling to a pressure below said desired container pressure of less than 10* of mercury, and a saturated vapor pressure below 10* mm. of mercury, when cooled to C., and valve means in said second pipe means for controlling the rate of flow of said fluid to said ejector, whereby when said pumping'fluid is injected into said pump body, high vacuum is attained.

3. A high vacuum pump apparatus including a primary pump, said pump apparatus comprising a pump body having one end connected with a container to be exhausted, a primary vacuum duct opening into said pump body, first pipe means connecting said duct with said primary pump, an ejector opening into a portion of said pump body located between said container-connected end thereof and the opening of said primary vacuum duct, means for cooling said portion of the pump body, a vessel for holding a pumping fluid invapor form at ambient temperature, said pumping fluid having at said temperature a saturated vapor pressure higher than 1 mm. of mercury, which pressure can be reduced by cooling to a pressure below said desired container pressure of less than 10- mm. of mercury, and second pipe means connecting said vessel with said ejector, said ejector directing the stream of said pumping fluid in a direction away from the container to be exhausted, and said cooling means including a liquid cooling agent boiling under atmospheric pressure at a temperature low enough to reduce the saturated vapor pressure of said pumping liquid fed from said ejector to below 10* mm. of mercury, thereby producing in said cooled portion of the pump body a saturated vapor pressure of the pumping fluid lower than the vacuum pressure to be obtained.

4. Apparatus according to claim 1 wherein said cooling means comprises boiling nitrogen as a cooling agent.

5. Apparatus according to claim 4, wherein said pumping fluid is Freon.

6. Apparatus according to claim 4, wherein said pumping fluid is carbon dioxide.

7. Apparatus according to claim 4, wherein said pumping fluid is water vapor.

8. Apparatus according to claim 1, further including 5 an auxiliary vessel, detachably secured to said pump opening and closing said auxiliary vessel, said auxiliary y, Coding means enclosing Said auxiliaryflessel, means vessel being provided with flange means for attachment to communlcatiflg Said auXilialY 165551 With Sald P p body said second pipe means in place of said primary vessel.

for allowing the passage of condensed vapor from said pump body to said auxiliary vessel, valve means for 5 No references cited. 

